As early as 1978 the assignee of the subject patent application proposed the use of a continuous, roll-to-roll process for the deposition of successive layers of amorphous semiconductor alloy material in order to fabricate n-i-p type photovoltaic devices in mass production. In such a fabrication process, an elongated web of substate material (typically 1000 feet in length, 14 inches wide and perferably formed of stainless steel or some other metallic material), provides both a rigid base upon which the successive layers of semiconductor alloy material can be deposited and the lower electrode of the photovoltaic device. This continuous fabrication process formed the basis of patentable inventions in commonly assigned U.S. Pat. Nos. 4,400,409 for "A Method of Making P-Doped Silicon Films And Devices Made Therefrom" and 4,410,588 for "Continuous Amorphous Solar Cell Deposition And Isolation System And Method". The use of an elongated metallic web of substrate material provides the ability to continuously deposit both the multiple layers of semiconductor alloy material and the upper transparent electrically conductive electrode prior to performing the division of the layers of semiconductor alloy material and electrodes into a plurality of smaller area photovoltaic segments. However, this roll-to-roll process translates into an economic advantage (as compared to batch processing) only if the small area division of the large area photovoltaic cells can be accomplished without patterning or other processing of the lower electrode prior to the initiation of the deposition process. This constraint is important because preliminary processing of the lower electrode would otherwise place the roll-to-roll, continuous web technology in the same economic and technological quagmire as techniques for batch processing thin film photovoltaic cells of relatively small area (such as 1 square foot devices) on glass substrates (wherein the light incident electrode is the first layer deposited and is immediately divided into a multitude of small area segments prior to the deposition of the layers of semiconductor alloy material or bottom electrode).
Therefore, until the assignee of the instant invention developed a substantially 100 percent effective method for eliminating short circuit defects (shunts) from elongated webs of photovoltaic cell material, which method was performed after the depositin of both the multiple layers of semiconductor alloy material and the upper electrode, the thin film photovoltaic industry split into two groups. The first group employed batch process techniques for fabricating large area photovoltaic modules. It was the position of this group that since both the batch processing and the continuous processing of photovoltaic cells requires the initial division of large area photovoltaic cells into small area segments and since batch processing required less expensive capital equipment for the fabrication of those cells, it made sound economic sense to utilize said batch processing techniques in preference to continuous processing techniques.
The second group advocated continuous procesing methodology for the production of large area photovoltaic cells. The rationale employed by the continuous processing group was clear. Since the ultimate goal of the photovoltaic industry was to compete in a cost competitive manner with all other depletable energy resources such as gas, coal and oil, the only way to be cost competitive is to generate photovoltaic power at a consumer cost per peak watt which is less than the cost of conventional power generation per peak watt. Thus, not only was it necessary to constantly increase the photoconversion efficiency of photovoltaic cells, but it was equally important to decrease the cost of fabricating those cells. Due to the fact that technological advances in the quality of discrete layers of semiconductor alloy material from which the photovoltaic cell is fabricated or in the design structure of said photovoltaic cell can be applied, with equal advantage, to both continuous and batch processing technology, it became readily apparent that the major economic advantages must be present in the fabrication technique itself.
It is to the end of reducing production costs so as to have the capability of competing on a cost competative basis with depletable energy resources that the instant invention is directed. More particularly, the subject inventors have formulated a process for fabricating large area photovoltaic cells on an elongated web of substrate material upon a first surface of which the layers of semiconductor alloy material as well as the upper electrode may be continuously and sequentially deposited without interruption for small area subdivision. The subdivision of the large area photovoltaic cells into small area photovoltaic segments is accomplished only after the aforementioned deposition process has been completed; and said subdivision includes the electrical interconnection of those small area photovoltaic segments in parallel to provide a high current, low voltage output. Previously, such continuous processing could not have excluded the possibility of catastrophic failure due to the presence of short circuit, current shunting defect paths (low resistance pathways through the thin film layers of semiconductor alloy material) whereby electric current collected at the electrodes flows through the current shunting path in preference to an external load. However, with the advent by the assignee of the subject patent application of a substantially failsafe short passivation procedure, as outlined in commonly assigned U.S. Pat. No. 4,729,970 issued Mar. 8, 1988, both patent (overt) defects, as well as latent defects, present in the layers of semiconductor alloy material and upper electrode can be isolated and passivated. Therefore, it becomes possible, for the first time, to produce large area photovoltaic modules, which do not require the initial step of patterning the bottom electrode thereof. And since a substantially 100 percent yield from the large area photovoltaic cell is a certainty, such cells can be utilized to provide a low voltage, high current output without fear of catastrophic failure. Further, by electrically interconnecting a plurality of said large area photovoltaic cells in series, the voltage therefrom can be added so as to provide any desired voltage-current combination. Just as important, however, is the ability to utilize the continuous fabrication technique, as specified hereinabove, in a manner which makes full use of the cost saving advantages inherent therein. No intermediate processing steps are required and the inherent economies of continuous cell fabrication can be utilized.
These and other objects and advantages of the instant invention will become apparent from a careful perusal of the drawings, the detailed description of the invention and the claims which follow.